In the discharge of his duties hereunder, the Secretary of the Interior, through the National Park Service, is authorized to employ, in his discretion, by contract or otherwise, landscape architects, architects, artists, engineers, and/or other expert consultants in accordance with the usual customs of the several professions and that expenditures for such employment shall be construed to be included in any appropriations hereafter authorized for any work under the objectives of sections 430y to 430z–3 of this title.